Especially after a first warning, I'm not familiar with any that would not dispatch that bull on the spot, without hesitation. It's pretty much rancher's law that all are aware of and none would take personally if utilized.
Keeping one's own bulls, with kindly attitude adjustments as needed, has its advantages.
How are the cows housed and how many? The most common option is to keep valuable cows secure from drifters. If one bull is getting to them then so can another.
Usually, if a herd of cows is worth the bother of AI then they're usually worth protecting from drifters. Now, if another's bull is coming in and destroying barns and stuff to get to cows that's a different story, but still, either way, the bull owner is usually liable and no bull owner wants to foot that bill, and most would even dispatch their own bull if needed not only to avoid that expense but to keep some decent honor and respect among neighbors if it even happened once, no less 2x.
But if the land and herd belong to grandpa and he sees no issue with the neighbors bull or the loss in profit, then it seems like the lower profit margin will need to be accepted. A really tough spot to be in for you guys. Securing the cows one way or another seems the only other option to keep profits up and grandpa happy.
Keeping one's own bulls, with kindly attitude adjustments as needed, has its advantages.
How are the cows housed and how many? The most common option is to keep valuable cows secure from drifters. If one bull is getting to them then so can another.

But if the land and herd belong to grandpa and he sees no issue with the neighbors bull or the loss in profit, then it seems like the lower profit margin will need to be accepted. A really tough spot to be in for you guys. Securing the cows one way or another seems the only other option to keep profits up and grandpa happy.